Since generation installations for renewable energy make a great contribution to the amount of energy produced for the public electricity grid, there is a desire to limit the extent to which energy is fed in by these installations as necessary in order to stabilize the electricity grid for example in the event of low power consumption by the connected loads, or to keep available a power reserve that can be called up at short notice. One possibility for limiting the feeding-in, that is to say for derating, consists in controlling the installation such that it feeds only a predefined percentage of the power available at the optimum operation point MPP (Maximum Power Point) into the connected electricity grid; another possibility consists in derating by a fixed absolute value, for example a predefined percentage of the rated power. In order to fulfill this predefinition optimally, it is necessary to know the maximum possible power of a generator of the installation. Since this power may change greatly over the course of time for example due to varying irradiation in the case of photovoltaic generators, it is necessary to regularly determine the present MPP power value anew. For this purpose, it is necessary to trace a characteristic curve of the generator over a sufficient range which contains the MPP or allows an estimation of the position of the MPP by extrapolation. In order to analyze the generator characteristic in this range, it may be necessary to cause the power fed in from the installation to deviate in part significantly from the derated power.
The document DE102010038941A1 discloses a network of a plurality of inverters operated via a common controller in such a way that one of the inverters is operated at the MPP, while the other inverters adapt their power such that the total power of the network corresponds to a predefined value. This solution requires a high communication overhead between the inverters involved and the common controller. Moreover, particularly in the case of derating to a very low derated power, it is not always possible to operate an installation of the network permanently at the MPP.